The present invention relates to nuclear magnetic resonance (NMR) imaging systems and, more particularly, to a novel circuit for speeding up the rise and fall times of current pulses utilized to generate gradient magnetic fields in such systems.
It is now well known that NMR imaging and/or spectroscopy systems require at least one power amplifier for each magnetic gradient field direction utilized. These gradient power amplifiers provide the current which generates the magnetic gradient fields, typically in the X,Y and Z dimensions of a Cartesian coordinate system, as necessary to obtain desired spatial resolution. Typically, the gradient power amplifiers are modified forms of linear high-fidelity audio power amplifiers, which typically generate current pulses in the 100-200 ampere range; the relatively good linearity, rise times and fall times of these amplifiers are obtained by the application of relatively high voltages and feedback to output stages containing as many as 100 bipolar transistors. These power amplifiers are relatively inefficient (having typical efficiencies of less than 15%). As higher imaging speeds are utilized, greater electrical stress is applied to existing gradient power amplifiers, as faster rise times require greater voltages (across the same gradient coil inductance) and so increasingly higher voltages and more power dissipation are all required. It is therefore highly desirable to provide a current amplifier circuit, preferably capable of being added to an NMR system in addition to, and between, an existing gradient power amplifier and an associated gradient coil, for providing the faster pulse current waveform rise and fall times necessary for higher-speed imaging use.
One such speed-up circuit is described and claimed in U.S. application Ser. No. 07/407,180, filed Sept. 14, 1989, assigned to the assignee of the present invention and incorporated herein in its entirety by reference; this speed-up circuit is relatively expensive and requires very large spatial volume. Additionally, the circuit does not allow a DC current to flow, either for shimming purposes or for providing a long-time-constant pulse, if required. This lack of precise waveform control is also desirably overcome in any new high speed gradient circuit. Such a circuit would operate with the inductance (about 1 mH) and resistance (on the order of 1 ohm) of a typical conventional whole body gradient coil, and will produce a 1 Gauss/cm gradient field strength at a coil current I.sub.L flow of about 100 amperes. Note that, ignoring resistance, a gradient strength of 1 G/cm will result in about 400 microseconds with an application of 250 volts to the coil. For snapshot imaging, in which an entire image is a acquired in 30 milliseconds or less, gradient strengths on the order of 3 G/cm, with rise times less than 100 microseconds, are required; use of voltages greater than 3 KV and currents of about 300 A are necessary to achieve such speed. Accordingly, a gradient magnetic field speed-up circuit allowing precise control during ramp waveform portions and providing for DC shim currents and the like, is highly desirable.